The invention relates to a process for producing metal-matrix composite materials made up of at least one portion of magnesium or of a magnesium alloy and at least one production step in which thixomolding takes place.
The material magnesium, due to its low elastic modulus, high coefficient of thermal expansion, and lack of wear resistance, cannot be easily used for certain applications, such as for pistons in motor vehicle engines or other assembly components, especially of engines. The indicated properties can however be beneficially influenced by the material being reinforced by means of a second, usually much more solid, and harder phase. Ordinarily ceramic or carbon-based short or long fibers or particles are used for this purpose. They can be infiltrated in melting metallurgical production, either in the form of a porous mold body (so-called preform) which is infiltrated with the liquid metal melt, or in the case of particles, added also by stirring into the metallic matrix. Another possibility for reinforcing a metallic material by fibers or particles consists in self-formation or also “in-situ” formation of the reinforcing component. In addition to the indicated melting metallurgical processes, metallic composite materials can also be produced by powder metallurgy.
When using preforms as the infiltratable mold bodies, squeeze casing has been established as the preferred casting process. In this connection, at somewhat lower mold filling speeds, but at somewhat higher pressures than in classical die casting, the molten metal is squeezed into the porous fiber or particle bodies. An almost pore-free composite material with closed fiber-matrix linkages is produced.
When stirring in, ordinarily ceramic particles as loose charge material are supplied to the moving metal melts by trickling or blowing in. Composite material melts of this type can be cast in the form of castings or bars. In the in-situ process the composite material is formed by a reaction between two or more alloying elements of the metallic matrix or phases of the overall system, generally with the formation of a new, generally intermetallic phase.
Production and characterization of the Mg—Mg2Si system have been repeatedly described. Reference is made for example to the disclosure of DE 41 25 014 A1. The formation of an intermetallic phase for the purpose of reinforcing can be assigned to the in-situ process. Generally this takes place by infiltration of Si particle-containing fiber preforms or by precipitation of primary magnesium silicides from hypereutectic Mg—Si alloys. While coarse, block-shaped Mg2Si precipitations form during primary precipitations after falling below the liquidus line, the Mg2Si in the reactive conversion of pure Si in a preform spheroidizes globally. Eutectically precipitated Mg2Si in turn generally shows the characteristic “Chinese script” structure.
DE 101 35 198 A1 describes a process for producing magnesium alloys by thixomolding, which alloys in addition to other elements can also contain a portion of silicon.
In the thixomolding process the metallic material is supplied as a granulate to the thixomolding machine and moved in the direction of the spray diffuser within the heated cylinder by a screw conveyor. Under the action of shear forces and the temperature which is between the liquidus temperature and the solidus temperature of the metal, it partially liquefies, while the remaining solid portion spheroidizes globally. The behavior of the thixotropic material is structurally viscous. i.e. the viscosity decreases with increasing shear action. Thixomolding is suitable especially for producing very thin-walled components with high dimensional stability, since as a result of the favorable temperature level between the liquidus line and solidus line hardly any shrinkage and warping phenomena occur.
The disadvantages of the aforementioned process routes for producing metallic composite materials in the case of preform infiltration lies in the complex plant technology, limited shaping capacity, fiber content of the preforms, and its high cost level. Complex geometries at present can hardly be accomplished or only at increased technical input and financial cost, so that net shape production of fiber- or particle-reinforced components by infiltration is hardly possible at present. This generally results in relatively high working effort which in use of ceramic hard phases as reinforcement is difficult and costly, since for example working of a body reinforced with SiC fibers or Al2O3 fibers is possible only by means of diamond-coated tools. Moreover the infiltration capacity of preforms with high fiber and particle content in a classic die casting is not easily given, preferably the squeeze casting process is used for this purpose, for which in turn special casting systems are necessary. The difficulties which can arise in infiltration by means of die casting are caused predominantly by the high filling rate of the process and the low pressure which can be applied over the melt as a result of the small gate. But this is necessary to overcome the normally very low wetting tendency between the metallic melt and the ceramic mold body. In addition, the preform must be heated distinctly above the melting point in order to avoid premature solidification of the melt on the fiber body.
The process of stirring-in is reserved first of all to the particulate reinforcements, since the use of fibers can lead to a major increase of melt viscosity which makes a uniform distribution of the fibers very difficult or even impossible. In the case of particles, the stirrer result is dependent on the particle size used, the stirrer rpm and the temperature. Inadequate parameter selection can lead to agglutination, scouring of particles into the slag, or their sedimentation on the crucible bottom. If the particles and melts are a reactive system, under certain circumstances due to the long contact time between the two phases conversion reactions on the interfaces occur, which result in damage to the particles. An example of this is for instance the magnesium/aluminum oxide system, here magnesium oxide and aluminum are formed in the reaction between the two partners with the decomposition of the particle substance.
It is an object of this invention is to make available a process for producing metal matrix composite materials of the initially mentioned type, which enables production of lightweight metal composite materials especially for use in temperature-stressed components, which is more variable and economical than the existing processes and avoids the disadvantages associated with them.